1. Field of the Invention
The present invention relates to a device for storing pressurized liquefied gas, notably ammonia or chlorine, and for distributing said gas at a high rate in the form of a high purity gas. A high rate is understood to mean a rate greater than or equal to 1 kg/h and preferably greater than 10 kg/h.
2. Related Art
Some industries, such as those producing semiconductors, solar cells or optical fibers are confronted at the present time with increasing requirements for high purity gas at various stages of production. Some of these gases such as HCl, Cl2, HBr, N2O, NH3, WF6, BCl3, 3MS, to mention only a few of these, are liquefied at ambient temperature and on account of this present distribution difficulties. These difficulties are directly linked to their pressure and/or their rate of use.
A liquefied gas is composed of two liquid and gaseous phases in equilibrium with each other. This equilibrium implies that at a given temperature, a liquefied gas has a well defined pressure and that this pressure varies as a function of temperature according to a relationship that is unique to each gas. In point of fact, the pressure increases as the temperature increases and conversely.
When the gaseous phase is drawn off from a liquid gas vessel, part of the liquid should be converted into gas so as to regenerate gas as it is used in order to maintain equilibrium. The liquid then starts to boil using available energy (typically energy from the external environment surrounding the vessel). The greater the draw-off rate the greater the energy requirement and the more the liquid boils violently and in this way creates a high risk of entraining droplets loaded with impurities in the gas phase. These droplets not only contaminate the gas but also accelerate corrosion phenomena and bring about instabilities in flow regulation and pressure measurements. If the available energy is not sufficient to turn the liquid to gas and in this way to generate the vapor phase, the temperature (and therefore the pressure) falls since equilibrium must be maintained.
In the case of pressurized liquefied ammonia (the reasoning would be identical with another gas) vaporization of the liquid ammonia phase at a temperature of 20° C., its vapor pressure being 8.7 bar, is accompanied by extraction of heat (QvapNH3) by approximately 250 kcal per kilogram of ammonia. Thus vaporization of ammonia at a rate of 100 kg/h corresponds to the extraction of 25000 kcal/h.
In an ISO vessel of 20000 kg of ammonia, this heat extraction has the effect of reducing the temperature (T) of ammonia by ΔT=QvapNH3/MNH3 CpNH3, MNH3 being the total mass of ammonia in the ISO-vessel and CpNH3 its specific heat at its vapor pressure: CpNH3=1.1 kcal/kg. ° K at 8.7 bar. Also, vaporization of 100 kg/h of ammonia will lower its temperature by approximately 1.1° C. per hour (ΔT=25000/20000×1.1).
It is thus possible to reach extremely low temperatures. The tendency in industry is to require higher gas flow rates and larger vessels, which increases cooling problems. By using the largest pressurized liquefied gas vessels, support and maintenance of many small vessels is eliminated and space is saved. Moreover, the frequency at which the vessel is changed is reduced, in this way reducing the risk of more frequent gas escapes during the steps of connecting and disconnecting vessels.
This cooling results in a lowering of the gas vapor pressure. For example, for ammonia, the vapor pressure will not exceed 6.3 bar at 15° C., 4.3 bar at 0° C. and 2 bar at −20° C.
In order to keep the gas pressure constant at the point of use it is thus necessary to keep its temperature constant. For this, it is necessary to provide a quantity of heat at least equal to the production of cooling (Qvapgaz).
An external supply of energy by heating makes it possible to limit cooling and the pressure drop observed. Several solutions may then be envisaged.
A first solution currently used consists of heating the gas vessel over all its height and keeping a constant heating temperature greater than ambient temperature. This solution requires complete tracing of the installation downstream of the vessel since the gas leaves the vessel at a temperature greater than ambient temperature. In point of fact, by applying such a solution, the pipeline for transporting gas at ambient temperature would create condensation and therefore bring about the presence of liquid in the pipeline, which is not acceptable since this would notably bring about heterogeneities in the flow rate of the product.
In patent application EP1538390 an induction heating system is described for a bottle containing liquefied gas. Patent application EP1298381 describes a device for heating a pressurized liquefied gas where the heating device is in permanent contact with said vessel and consists of electrical heating means. Patent application US 2002/0124575 describes a method for controlling the temperature of a pressurized liquefied gas situated in a storage device heated by a heat source outside said storage device.
However, such devices are still not satisfactory. In point of fact, the gases used in the applications described above are corrosive and flammable, while with an electrical heating system the risk of an explosion linked to escapes of such gases cannot be excluded. Also, it is impossible for example to use a sprinkler cooling system in case of necessity on account of these electrical systems. Risks of burns, corrosion of the storage vessel and electrocution are also highly probable. In addition, cleaning, maintenance and replacement of equipment in case of breakdowns are complex, risky and costly.